If the fate of the world in Attack on Titan required Eren Yeager and Mikasa Ackerman to become lovers, then you'd get new manga You, the One and Only, and the Seven Billion Grim Reapers. Such a plot point would have undoubtedly made Attack on Titan more popular (if that's even possible), so fans can expect big things from this up and coming series.
Attack on Titan is set in a world where certain humans can transform into the gigantic humanoids known as Titans. Upon the so-called Titan Shifters' transformation, these humans become enclosed in the nape of the Titan's neck, which acts like a vessel cockpit, as they control the massive creature's movements from within. The origin of Titans can be traced back to Ymir Fritz and her descendants, known as the Subjects of Ymir, who later reproduced to expand the bloodline. In this world, defeating these massive creatures has nothing to do with love. Instead, it's the ability to navigate in omni-directional mobility gear, which grants wearers the ability to move in ways similar to Spider-Man's web-slinging, but with deadly purpose.
In Seven Billion Grim Reapers, there are gigantic, powerful beings called RUM that are not as grotesque as Titans but still began as humans. These humans can also transform into RUM on command and appear inside the humungous body upon completion, wherein they control the massive creature. But rather being enclosed in the nape of the RUM's neck like Titans, they appear behind the eyes. Meanwhile, unlike those who just inherited the Power of the Titans, RUM are the victims of a horrific genocide in the future who somehow come back to life, imbued with this new ability. But the twist is that the further Seita Tooki and Awayuki Kujou progress in their relationship, the more powerful Seita becomes. Because of this, the RUM are naturally compelled to ensure their relationship fails at all costs.
Unlike Attack on Titan's Eren Yeager, Seita can't transform into his world's version of a Titan, nor does he don specialized gear that makes maneuvering around RUM easier. He just essentially possesses a power that cancels out the source of the RUM's abilities. In fact, a RUM who's rooting for Seita and Awayuki even goes so far as granting Seita the power to stop her kind. This enigmatically helpful RUM also provides certain information to ensure Seita succeeds, like the number of days until the next invasion will transpire. The first time this occurs, she informs Seita that he won't win unless he kisses Awayuki.
Awayuki leads an incredibly sheltered life, believing her first kiss must take place on her wedding day. So navigating such a difficult predicament, fraught with many emotional landmines, proves to be a seemingly impossible endeavor for Seita. When Seita eventually clues her in, they eventually come to an absurd compromise that transpires at an equally ridiculous time: they have a "promise" ceremony during the actual invasion.
One of the few things Attack on Titan is missing is romance throughout the series, with many fans wanting to see a more complex relationship evolve between Eren and Mikasa. Of course, romance has the power of making or breaking series like this one, and must be done tactfully. In Seven Billion Grim Reapers, that's the case, with the connection between the RUM threat and the protagonists' romantic tension adding a compelling dynamic to the series and its characters, making it even more worthwhile than the already thrilling stakes would suggest.